How can I make a shadow disappear in Cycles?
This affects the object casting the sadow not the lamp.

A slightly simpler solution exists: you can set the object you want not to cast shadows not to interact with shadow rays. You can then render your animation with and without the shadow. Finally you mix the two animations to control the fading.

The parameters are in Object > Ray Visibility > Shadow. See this qustion for more info about this:

How to make a cycles light emisson object invisible to the camera?